The present invention relates to a passive infrared detector used in a burglar alarm system for detecting an intruder.
A passive infrared detector detects a moving object that radiates infrared rays. The passive infrared detector has an optical system, e.g. a reflecting mirror or a lens, for determining an area for detecting a moving object. The passive infrared detector further has a pyroelectric element placed at the focus position of the optical system, and a signal processing circuit. When the output level of the pyroelectric element exceeds a threshold value, the signal processing circuit outputs an alarm signal indicating that a moving object has been detected.
Hitherto, Japanese Utility Model Application Unexamined Publication (KOKAI) No. 1-46890, for example, discloses an optical system of a passive infrared detector that enables the extent of the field to be 360 degrees in circumference. However, the conventional system has a very complicated structure and also requires a reflecting mirror difficult to design. Furthermore, when it is desired to narrow the monitoring zone in a certain direction on account of the installation place, the conventional system must employ a technique known as mirror masking, in which a portion of the reflecting mirror that corresponds to the certain direction is masked. Thus, the conventional system involves a troublesome operation and is not favorable from the viewpoint of effectively utilizing the reflecting mirror.
To solve the above-described problems, the present applicant proposed in Japanese Patent Application No. 9-54401 a passive infrared detector having sensor units comprising a plurality of reflecting mirrors that form monitoring zones and sensing elements placed at the respective focus positions of the reflecting mirrors. The passive infrared detector enables the extent of the field to be approximately 360 degrees in circumference with a simple arrangement and also allows the field range in the vertical direction to he partially adjusted.
However, the proposed passive infrared detector as described above involves some problems to be solved in terms of specific structures for adjusting the field range in the vertical direction and for partially adjusting the field range in the vertical direction.